Gastrointestinal bleeding or gastrointestinal hemorrhage describes every form of hemorrhage (loss of blood) in the gastrointestinal tract, from the pharynx to the rectum. It has diverse causes, and a medical history, as well as physical examination, generally distinguishes between the main forms. The degree of bleeding can range from nearly undetectable to acute, massive, life-threatening bleeding.

Gastrointestinal bleeding can range from microscopic bleeding, where the amount of blood is such that it can only be detected by laboratory testing, to massive bleeding where bright red blood is passed and hypovolemia and shock may develop. Blood that is digested may appear black rather than red, resulting in "coffee ground" vomit or stool.

Upper gastrointestinal bleeding is characterized by hematemesis (vomiting up blood) and melena (tarry stool containing altered blood). About half of cases are due to peptic ulcer disease. Esophagitis and erosive disease is the next most common causes.In those with liver cirrhosis 50-60% of bleeding is due to esophageal varices.Approximately half of those with peptic ulcers have an H. pylori infection.Other causes include: gastric or duodenal ulcers, Mallory-Weiss tears, cancer, and angiodysplasia.

A number of medications are found to cause upper GI bleeds.Lower gastrointestinal bleeding Lower gastrointestinal bleeding is typically from the colon, rectum or anus.Causes include: hemorrhoids, cancer, angiodysplasia, colitis, and aortoenteric fistula.It may be indicated by red blood per rectum, especially in the absence of hematemesis. Isolated melena may originate from anywhere between the stomach and the proximal colon.